Stock PSU cables aren't not, let me repeat, ARE NOT pretty high-end cables. They don't even start to come close. You need to buy 16AWG cables from this guy.
http://www.shakmods.com/shop&&#!/~/product/category=3316088&id=37642162
Those are "HIGH-END" cables. The fact that the cables started to melt at the cube end suggests they were plugged in correctly or they simply are cheap quality cables.
Seasonic PSUs are pretty high end in terms of quality. I am pretty sure the PCIe cables on these 1250W units are 16AWG. They are really thick. I don't think the issue here is with the wires. I still have a bunch of Neptune cubes using Seasonic 1250W PSUs for months without any signs of burning but this Titan cube has burn marks after just 5 days of mining. I am certain that everything was plugged in correctly. Because of all the issues people have had with their Neptune's, I triple check all my connections when I set up equipment. Requiring third party cables and terminals is silly. They really should have provided the PSUs and cabling if they were going to go out of standard spec considering how much customers paid for these miners.